An image forming apparatus such as a copier, a facsimile machine or a printer includes an image forming unit for forming an image on an image carrier (e.g. photoconductive drum), a transfer unit for transferring a toner image on the image carrier onto a sheet as an example of a recording medium and a fixing device for heating and fixing the toner image transferred onto the sheet to the sheet.
There is known a fixing device to which an electromagnetic induction heating (IH) method capable of quick heating and high efficiency heating is applied. In the electromagnetic induction heating method, an induction current is induced in a fixing roller and a fixing belt by a magnetic flux generated by the flow of a high-frequency current in an induction coil and Joule heating (induction heating) is caused in the fixing roller and the fixing belt. A toner image is fixed onto a sheet (recording medium) by this Joule heat.
A technology for suppressing excessive temperature increases of a fixing belt and a fixing roller in a fixing device of an electromagnetic induction heating type is, for example, known from Japanese Examined Patent Publication No. 2011-123409. A fixing device of Japanese Examined Patent Publication No. 2011-123409 includes a fixing belt configured to be induction-heated by a magnetic flux generated by a coil, a pressure roller configured such that a nip portion is formed between the fixing belt and the pressure roller, arch cores and side cores configured to form a magnetic path together with the fixing belt, a center core arranged between the arch cores and the fixing belt when viewed from the magnetic path and a magnetic shielding plate attached to the outer surface of the center core. The magnetic shielding plate comes to be located in the magnetic path with the rotation of the center core and suppresses excessive temperature increases of the fixing belt and the fixing roller in sheet non-passage areas by shielding or suppressing the magnetic flux in accordance with the sheet non-passage areas. Further, a technology for cooling an electromagnetic induction heating unit of a fixing device by cooling air is disclosed in Japanese Examined Patent Publication No. 2011-227445.